DESCRIPTION: The proposed research deals with the control of head movements by the vestibular system. Such movements play an essential role in postural reflexes and the orientation of cranial sense organs, but we do not know how these movements are organized within the central nervous system. Abnormalities in vestibulocollic function in man are associated with deficits in postural regulation and are common sequelae of aging, contributing to dependence, social isolation, and depression. To understand vestibular control of head movements we need to know the functional components of the vestibular nerve and the role each component plays in the control of posture and gaze. In experiments already completed or underway I am characterizing the morphological classes of vestibular afferents in P. scripta and their presynaptic hair cells. The proposed research will extend this analysis of vestibular mechanisms. I Will (1) reconstruct and quantify the architecture of vestibular afferents that have been completely filled by intracellular injections of biocytin and characterized physiologically; (2) use serial section light and electron microscopy of extracellularly filled afferents to identify the central synaptic contacts between each afferent type; (3) use double labeling electron microscopy to characterize the synaptic contacts between each afferent type and the various populations of vestibulospinal neurons. The object of the experiments is to learn the structural mechanisms by which signals from each afferent type are transformed at their first central synapse. This research, combined with work in progress, will contribute to our understanding of vestibular mechanisms in three ways. First, it will greatly expand our understanding of information channels in the vestibular nerve. Second, it will provide an important addition to our understanding of descending vestibular control. Third, it will help us understand how head and neck reflexes are coordinated to achieve postural and visual stability in the world of constant motion.